To do large-scale quantum information processing, it is necessary to control the interactions between individual qubits while retaining quantum coherence. To this end, superconducting circuits allow for a high degree of flexibility. We report on the time-domain tunable coupling of optimally biased superconducting flux qubits. By modulating the nonlinear inductance of an additional coupling element, we parametrically induced a two-qubit transition that was otherwise forbidden. We observed an on/off coupling ratio of 19 and were able to demonstrate a simple quantum protocol.
Niskanen, A., Harrabi, K., Yoshihara, F., Nakamura, Y., Lloyd, S., & Tsai, J. S. (2007). Quantum Coherent Tunable Coupling of Superconducting Qubits. Science, 316(5825), 723-726. https://doi.org/10.1126/science.1141324